Formation

Several mechanisms have been proposed for the Moon's formation 4.527 ± 0.010 billion years ago,[f] some 30–50 million years after the origin of the Solar System.[13] These included the fission of the Moon from the Earth's crust through centrifugal force[14] (which would require too great an initial spin of the Earth),[15] the gravitational capture of a pre-formed Moon[16] (which would require an unfeasibly extended atmosphere of the Earth to dissipate the energy of the passing Moon),[15] and the co-formation of the Earth and the Moon together in the primordial accretion disk (which does not explain the depletion of metallic iron in the Moon).[15] These hypotheses also cannot account for the high angular momentum of the Earth–Moon system.[17]
The prevailing hypothesis today is that the Earth–Moon system formed as a result of a giant impact: a Mars-sized body hitting the newly formed proto-Earth, blasting material into orbit around it, which accreted to form the Moon.[18] Giant impacts are thought to have been common in the early Solar System. Computer simulations modelling a giant impact are consistent with measurements of the angular momentum of the Earth–Moon system and the small size of the lunar core. These simulations also show that most of the Moon came from the impactor, not from the proto-Earth.[19] However more recent tests suggest more of the Moon coalesced from the Earth and not the impactor.[20][21][22] Meteorites show that other inner Solar System bodies such as Mars and Vesta have very different oxygen and tungsten isotopic compositions to the Earth, while the Earth and Moon have near-identical isotopic co positions. Post-impact mixing of the vaporized material between the forming Earth and Moon could have equalized their isotopic compositions,[23] although this is debated.[24]
The large amount of energy released in the giant impact event and the subsequent reaccretion of material in Earth orbit would have melted the outer shell of the Earth, forming a magma ocean.[25][26] The newly formed Moon would also have had its own lunar magma ocean; estimates for its depth range from about 500 km to the entire radius of the Moon.[25]
Despite its accuracy in explaining many lines of evidence, there are still some difficulties that are not fully explained by the giant impact hypothesis, most of them involving the Moon's composition.
In 2001, a team at the Carnegie Institute of Washington reported the most precise measurement of the isotopic signatures of lunar rocks.[27] To their surprise, the team found that the rocks from the Apollo program carried an isotopic signature that was identical with rocks from Earth, and were different from almost all other bodies in the Solar System. Since most of the material that went into orbit to form the Moon was thought to come from Theia, this observation was unexpected. In 2007, researchers from the California Institute of Technology announced that there was less than a 1% chance that Theia and Earth had identical isotopic signatures. [28] Published in 2012, an analysis of titanium isotopes in Apollo lunar samples showed that the Moon has the same composition as the Earth,[29] which conflicts with the moon forming far from Earth's orbit or from Theia. Variations on GIH may explain this data.